Seal for use in a volume displacement plethysmograph

ABSTRACT

Disclosed is a neck seal for use in a volume displacement plethysmograph for investigating pulmonary function of infants. The neck seal includes particulate material entrapped in a space between two layers of flexible sheets. The two flexible sheets have aligned openings to allow an infant&#39;s neck to be located therein. The sheets are sealed together around the openings and along a closed figure extending around the openings, thereby forcing the space closed. The seal becomes rigid when a vacuum is applied to the space.

[0001] THIS INVENTION relates to a seal suitable for use in a volumedisplacement plethysmograph for investigating pulmonary function,particularly of infants. In particular, the invention is directed to aneck seal.

BACKGROUND ART

[0002] Objective assessment of respiratory function plays an importantpart in understanding the physiology and pathophysiology of therespiratory system. Lung function is an indispensable tool for diagnosisand monitoring of respiratory disease states in adults and olderchildren, but has not gained wide acceptance in the management of infantlung disease. The primary difficulty in measuring infant lung functionis the inherent lack of cooperation requiring assessment during sedatedsleep and the use of a face mask. To date there is no widely appliedtechnique for the measuring infant lung function in the unsedatedinfant.

[0003] Some methods of monitoring respiration in an infant have beendeveloped for specific needs. Known devices for monitoring infant lungfunction can generally be classified as either invasive or non-invasivewith respect to the infant's airway.

[0004] Invasive monitoring devices include pneumotacchographs which areconnected to a sealed face mask, or spirometers similarly connected to aface mask. Traditional methods of volume and flow measurement with aface mask and/or pneumotachograph induce error through the effects oftrigeminal stimulation, increased dead space and resistive loading. Facemasks are poorly tolerated in unsedated infants and induce arousalespecially in light sleep. They are completely impractical in the awakeinfant.

[0005] In addition it is technically difficult to maintain a seal withthe infant for protracted periods of time, thereby limiting the abilityto acquire data dynamically during unpredictable respiratory events suchas apnoeas, sighs and hypopnoeas. These events are typically associatedwith desaturation, and understanding respiratory dynamics that surroundsuch events forms an important part of sleep medicine.

[0006] Non-invasive devices typically use bands to detect changes inchest and abdominal wall dimensions to monitor breathing. Such devicesare easier to operate technically, and are less disturbing to thesubject. However, these devices generally suffer from inaccuracy inmeasuring or interpreting lung volume changes.

[0007] Respiratory inductance plethysmography (RIP) has been used as onesuch non invasive measure of tidal volume and minute volume in infantsbut is compromised by complex and time consuming calibration techniques,though a simplified calibration has been recently disclosed. Apneumotachograph and face mask is still required to calibrate RIP tomeasure volume accurately and therefore can only be used in very youngor sedated infants.

[0008] A fundamental problem with RIP is the approximation of the infantrespiratory system as a two compartment model. In disease states chestwall motion is complex with subcostal and suprastemal recession beingtypical features. In infants with respiratory distress syndrome, chestwall recession in the inferior aspect of the chest may occur withexpansion in the upper portion of the chest, and it is unlikely that asingle RIP band can accurately measure thoracic volume changes in suchsituations. RIP has not been validated as a measure of tidal volume ininfants with lung disease, and it is known to be inaccurate in infantsunder 1.5 kg presumably because of variable chest wall compliance.

[0009] Constant volume plethysmography involves the insertion of theinfant in a sealed chamber and the application of a face mask to permitthe infant to breathe fresh air and to remove expired air. Occlusion ofthe airway at the mask results in respiratory efforts by the infant fora small number of breaths. This, in turn, compresses and rarefies thegas within the chamber. By measuring the pressure changes, and knowingthe volume of the chamber, Boyle's Law permits an estimate of the totalgas within the infant's lung at the time of occlusion. However, theprocedure is technically difficult, and the procedure is not suitablefor protracted periods of time, e.g. during sleep.

[0010] U.S. patent application Ser. No. 09/124926 describes an improvedplethysmograph for measuring infant lung function non invasively duringunsedated sleep without the need for a face mask.

[0011] It is an object of this invention to provide an improved sealparticularly, but not solely, suitable for use with that plethysmograph.

SUMMARY OF THE INVENTION

[0012] In one broad form, this invention provides a seal suitable forsealing the entry of a neck portion, limb or the like into a chamber,comprising

[0013] a pair of juxtaposed flexible sheets each having an openingtherein, the openings being aligned, the sheets being sealed togetheraround the openings and along a closed figure around, and spaced from,the openings, to define a closed space between the sheets;

[0014] particulate material in the closed space; and

[0015] an opening in one of the sheets to permit the closed space to beconnected to a vacuum pump or the like, whereby upon evacuation of airfrom the closed space, the seal adopts a substantially rigid form.

[0016] Typically, the flexible sheets are made of elastomeric material,such as latex rubber.

[0017] The neck seal is flexible to permit passage of a patient's headthrough the openings, but is stiffened in use to a substantially rigidform In this manner, if the neck seal is used in a plethysmograph, itdoes not substantially alter the constant volume of the plethysmographchamber during pulmonary monitoring operations.

[0018] The particulate material may suitably be particles or beads ofexpanded polystyrene.

[0019] The opening is suitably in the form of a spigot for ease ofconnection to a tube leading to a vacuum pump.

[0020] In order that the invention may be more fully understood and putinto practice, a preferred embodiment thereof will now be described withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a perspective view of a plethysmograph (with infant),having a neck seal according to one embodiment of the invention;

[0022]FIG. 2 is a schematic sectional elevation of the plethysmograph ofFIG. 1;

[0023]FIG. 3 is a schematic perspective view of a spirometer used in theplethysmograph of FIG. 1;

[0024]FIG. 4 is a perspective view of the neck seal of FIG. 1;

[0025]FIG. 5 is a sectional view along A-A of the neck seal of FIG. 4;and

[0026] FIGS. 6(a) and 6(b) are polysomnographic recordings from twodifferent infants.

DESCRIPTION OF PREFERRED EMBODIMENT

[0027] As shown in FIG. 1, a low body plethysmograph comprises a chamberin the form of a box 10 in which an infant is positioned from the neckdown. The sides of the box 10 preferably include transparent portions sothat the infant can be visually monitored. In the illustratedembodiment, the box is a rigid clear acrylic box constructed to internaldimensions of 180×320×500 mm to accommodate a range of infants. Aremovable top plate 10A serves as a base of a water sealed spirometer11, described in more detail below.

[0028] The box 10 is provided with a removable end wall 10C connected toa mattress tray 10D. The mattress tray allows easy access to the infantto aid settling and facilitates insertion of the infant within thedevice without disturbance.

[0029] The end wall 10C is in the form of a rectangular frame having aneck seal 20 therewithin to provide an airtight seal around the neck ofthe infant. (The neck seal is described in more detail below). Aperipheral gasket can be used to provide an airtight seal between theend wall frame 10C and the opposed periphery of the entrance to the box10.

[0030] As can be seen from FIG. 1, the infant's head remains outside ofthe box 10. This permits the infant to breathe room air easily, as wellas the application of therapeutic maneuvers to the infant.

[0031] Changes in chest and abdominal volumes of the infant duringrespiration are monitored using a spirometer 11 mounted to the top ofthe box 10. The spirometer 11 (described in more detail later) is influid communication with the interior of the box 10 via a spigot-likeport 12 which is preferably located directly over the chest portion ofthe infant. A water seal 13 is suitably provided around the spirometer11.

[0032] The construction of the plethysmograph is such that it maintainsa constant pressure on the infant, as opposed to constant volumechambers used in prior art devices. Although constant pressure chambershad been used previously in adult respiratory medicine, they met withvery limited success. The two principal technical problems with knownconstant pressure chambers were (i) the inability of the spirometer tomeasure small volume changes, and (ii) the maintenance of an airtightseal around the patient's neck. These problems are overcome by theplethysmograph illustrated in the attached drawings.

[0033] As shown in FIG. 3, the spirometer 11 used with theplethysmograph comprises an open-bottom cover or “bell” 14 which isconnected by radial arms 15 to a rotatably mounted shaft 16. The bell 14is typically of lightweight polystyrene (volume 120 mL, mass 16 g) andthe shaft 16 may be of carbon fibre. A counterweight 17 may also beconnected to the rotatable shaft 16, to balance the weight of the bell14. However, as the bell 14 is lightweight, only a small counterweight(if any) is required.

[0034] A linear potentiometer 18 is mounted to the shaft 16. Thepotentiometer is connected by wires 19 to electronic monitoringapparatus which is able to monitor the rotational position of shaft 16.

[0035] The side walls of the bell 14 are immersed in a reservoir ofwater 13 surrounding the port spigot 12, so that the water forms a seal.Further, two opposed walls of the bell 14 are shaped such that a linearrelationship exists between the change of volume within the bell 14above the water surface and the angle of rotation of the shaft. Thesewalls are of arcuate shape, concentric with the axis of rotation of theshaft 16. This radial pivoting design allows ease of transduction to anelectrical signal by the linear potentiometer 18 on the axis ofrotation.

[0036] The lightweight bell chamber 14 (optionally counterweighted)provides minimal static back pressure (typically less than 1 cm ofwater). The spirometer 11 is sensitive to very small changes inabdominal and/or chest volume of the infant. Typically, volume changesas small as 2 ml can be detected.

[0037] The lightweight spirometer 11 has a single degree of freedom of(radial) movement, thereby minimising phase errors. The illustratedspirometer was employed to measure volume displacements from the boxrather than a pneumotachograph because of the superior low frequencyresponse extending to 0 Hz.

[0038] Airtight electrical connections can be provided to permit passageof cabling into the box 10 for ECG, oximetry and RIP. A port at the sidecan also be provided for attachment of a calibrated gas syringe forspirometer calibration.

[0039] A circulating fan can be used to provide air mixing within thechamber through a silica gel cartridge and to ensure that recorded boxhumidity is controlled in the presence of skin losses. Box temperatureand humidity can be recorded with battery operated instruments withinthe chamber. Multichannel signal acquisition and storage is typicallyperformed by a personal computer.

[0040] The neck seal 20 is shown in more detail in FIGS. 4 and 5. Theneck seal satisfies the conflicting requirements of being a compliantmembrane to permit its application over the infant's head, yet beingrigid in use to maintain dimensional stability of the box 10 and therebypermit any volume change to be faithfully recorded by the spirometer.

[0041] The neck seal 20 comprises two layers of elastic material,preferably a natural latex rubber (170 μm mean thickness). The twolayers are sealed together around the periphery 21 of the neck seal.Holes 22 are suitably placed in the sealed periphery 21 to locate theneck seal with the frame 10C and/or the open end of the box 10.

[0042] A neck opening 23 is provided in both layers of the neck seal,and the two layers are sealed together at the annular portion 24 aroundthe neck opening. Several neck seals with holes of varying sizes from 30mm to 55 mm diameter may be provided to accommodate a range of infantneck circumferences.

[0043] Particulate material, such as small beads or particles ofexpanded polystyrene 25, are inserted between the two sealed layers ofthe neck seal. The particulate material may also be fibrous, stick-likeor any other suitable particle components. A spigot 26 is provided onthe outer side of the neck seal for connection to a vacuum pump or thelike.

[0044] In use, with no vacuum applied, the rubber neck seal is stretchedover the infant's head and placed around the infant's neck. At thisstage, the neck seal is still flexible. The spigot 26 is then connectedto the vacuum pump. When the air between the two layers of the neck sealis evacuated to a pressure of −10 to −20 kPa, the two layers of the neckseal compact the particulate materials therebetween and with each other,and form a substantially rigid wall. (That is, the neck seal wallremains rigid for the volume/pressure changes encountered in use).

[0045] The frequency response and phase characteristics of theplethysmograph were tested using a loudspeaker driven by sinusoidalsignals of variable frequency through a DC coupled amplifier. Boxacoustic capacitance was tested by the rapid and slow injection of 100ml of air while the sealed box pressure was recorded. Spirometerinertance was calculated by the introduction of a varying signal at thespirometer port 12 whilst the pressure was recorded at the point ofintroduction.

[0046] The spirometer 11 was calibrated by inserting known volumes ofair into the plethysmograph box 10 using a calibrated gas syringe priorto insertion of the infant. After monitoring devices were applied to theinfant a normal feed was given and the settled infant placed on thetray. Most infants were studied supine but decubitus position and proneposition were used successfully. The infant's head was delivered throughthe neck seal aperture which was stretched open by the operator.

[0047] Application of the vacuum then stabilises the seal in an airtightposition whilst conforming to the infants sleep position. Any leak atthe seal is apparent from a monophasic baseline drift on the spirometeroutput and data are discharged until the seal is re-established. (Thismay involve minor positioning maneuvers of the infant's head and neck).

[0048] The box temperature elevated 1.5 to 2 degrees Celsius aboveambient to a steady state with changes being less than 1.0 degree perhour. Gross body movement which induced artefacts upon the volume signalwere edited accordingly.

[0049] The capacitance, resistance and inertance of the describedplethysmograph and spirometer are shown below (measured with 21 normalsaline simulating infant volume). Acoustic Capacitance: 0.181. kPa⁻¹(0.018 1. cm H₂O⁻¹) Resistance: 0.021 kpa.1⁻¹ .s(0.21 cm H₂O 1⁻¹.s⁻¹)Inertance: 0.0015 kPa 1⁻¹ .s² (0.015 cm H₂O 1⁻¹ .s²)

[0050] Eight normal infants and seven infants with varying degrees ofairway and interstitial lung disease aged from term to three months weresuccessfully studied. Some infants were studied for over two hours, thelimiting variable usually being the timing of the next feed. Nocomplications were encountered and most infants settled spontaneously.Difficulties were experienced with some larger infants who weredisturbed by the restrictive space within the plethysmograph. One infantfailed to settle in the box due to the restrictive space and the studywas abandoned. All infants were fed, prepared and settled in the boxwithin one hour.

[0051] Polysomnographs (PSG) of infants studied in the VDP are presentedin FIGS. 6(a) and (b) illustrating the ability to record tidal volume,FRC changes and sigh volume in unsedated sleep.

[0052]FIG. 6(a) is a sample recording of a 2.2 kg infant of correctedage 36 weeks studied in the plethysmograph. An exaggerated form ofexpiratory braking is illustrated, the RIP is suggestive of respiratoryparadox. The vertical line shows lag in the RC expiration signalrelative to actual volume loss.

[0053]FIG. 6(b) is a recording of an infant with bronchopulmonarydysplasia at 41.5 weeks corrected age and weight 3.8 kg. REM sleep showsfluctuations in the baseline of the spirometer signal. This representsFRC variations in the order of 20 mls over the epoch displayed.

[0054] Note: All records show from the top down, EEG (C3/A2) EOG leads(LE, RE), rib cage (RC) and abdominal (AB) RIP signal, oxygen saturation(SaO₂) and volume displacement signal. Inspiration is up on both RIP andvolume signals. Arrows denote eye movement.

[0055]FIG. 6(a) illustrates how misleading conventional RIP can be inaccurately representing tidal breathing. The RIP channels appear onfirst inspection to be typical paradox. However, analysis of the volumechannel shows that this infant is adopting an exaggerated form ofexpiratory braking or breath hold. It can be seen that the paradoxicalinward chest wall movement is in fact partly due to expiration of theprevious breath.

[0056] The continuation of RC contraction beyond end expiration reflectsthe response of the compliant chest wall to the commencement of the nextinspiratory effort.

[0057] The described plethysmograph is suitable for monitoring tidalvolume, respiratory rate and changes in FRC in unsedated sleep ininfants with and without lung disease. It enables data to be obtained inREM and NREM sleep. The mechanical properties of the plethysmograph andspirometer are suitable for recording tidal breathing parameters ininfants. The spirometer could also be developed and used in moretraditional apparatus displaying adequate frequency response, resolutionand linear dynamic range for monitoring tidal breathing in infants. Thedifferentiated volume signal (flow) would be suitable for thedetermination of tidal breathing timing indices such as time to peakexpiratory flow/expiratory time (T_(PEF)/T_(E)). The advantages of thespirometer over a pneumotachograph are stability of calibration, reducedresistive load and low frequency response extending to 0 Hz. (Theinferior low frequency response of the pneumotachograph is due to verysmall pressure drops across the resistive element at low flows whichmake it less likely to accurately resolve slow FRC drifts seen insleep).

[0058] The neck seal 20 is comfortable, easy to apply and permitsnatural sleep as well as being mechanically stable. Mechanical stabilityensures volume displacements within the box are accurately representedby the spirometer without deformation of the neck seal. Minimal contactpressure is required on the neck of the infant in order to obtain a sealbecause of the very small static pressure of the spirometer.

[0059] The plethysmograph of this invention allows monitoring of FRCchange on a breath to breath basis over protracted periods of time thusallowing a quantitative measure of FRC instability to be made ininfants. (In order to show changes in FRC over relatively long periodsthe DC coupled mode should preferably be used). This data is otherwiseunobtainable with the washout/dilution techniques and with constantvolume plethysmography which tenders a static measurement of FRC. Theseknown techniques also require the use of a face mask andpneumotachograph. The use of face mask and pneumotachograph may alterFRC by the effects of resistive loading on timing indices, trigeminalstimulation and increased dead space.

[0060] The plethysmograph of this invention permits dynamic measuring oftidal volume and change in FRC without the use of a face mask. Thedevice is suitable for monitoring sleeping infants for prolonged periodswithout sedation. The device is simply and quickly calibrated.Furthermore, the interpretation of respiratory phenomena derived fromconventional non invasive monitoring is enhanced. The plethysmographcould also be used to investigate other dynamic respiratory mechanics ofthe sleeping infant. The “head out” construction also permits varioustherapeutic interventions to be monitored without orofacial stimulation.

[0061] The foregoing describes only one embodiment of the invention, andmodifications which are obvious to those skilled in the art may be madethereto without departing from the scope of the invention.

[0062] Although the spirometer and neck seal have been described withparticular reference to their use in the plethysmograph, they are ableto be used independently in other applications. For example, thespirometer may be used to measure small volume changes with highsensitivity in both physiological (e.g. lung compliance, limb occlusion)and non-physiological applications. Similarly, the neck seal can be usedin physiological (e.g. limb splinting) and non-physiologicalapplications.

What is claimed is:
 1. A seal, suitable for sealing entry of a bodyparty into a chamber, the seal comprising: a pair of juxtaposed flexiblesheets each having an aperture therein, the apertures being aligned toform an opening, the sheets being joined together around the opening andalong a closed figure extending around, and spaced from, the openings todefine a closed space between the sheets, wherein the sheets around theopening is sufficiently flexible to permit expansion of the opening; aparticulate material in the closed space; and a port in at least one ofthe sheets to permit gas to come into or out of the closed space,wherein upon evacuation of gas from the closed space, the seal adopts asubstantially rigid form.
 2. The seal of claim 1, wherein the flexiblesheets are made of an elastomeric material.
 3. The seal of claim 2,wherein the elastic material comprises latex rubber.
 4. The seal ofclaim 1, wherein the particulate material comprises beads of expandedpolystyrene.
 5. The seal of claim 1, wherein the port is in the form ofa spigot.
 6. The seal of claim 1, wherein, the openings aresubstantially circular.
 7. In a plethysmograph having a chamber adaptedto accommodate a patient therein in use with the patient's headpositioned outside the chamber, a seal operatively located around thepatient's neck and comprising: a pair of juxtaposed flexible sheetsjoined together around a common opening therethrough and along a closedfigure extending around, and spaced from, the opening to define a closedspace between the sheets, at least a portion of the sheets around theopening being sufficiently elastic to permit expansion of the openingadapted to allow the patient's head therethrough yet to close around thepatient's neck in a substantially airtight fit; a particulate materialin the closed space; and a port in at least one of the sheets to permitgas to come into or out of the closed space, wherein upon evacuation ofgas from the closed space, the seal adopts a substantially rigid form.8. The seal of claim 7, wherein the flexible sheets are made of anelastomeric material.
 9. The seal of claim 7, wherein the port is in theform of a spigot.
 10. The seal of claim 7, wherein the sheets are joinedtogether by sealing along an annular portion around the opening andalong a portion around the periphery of the sheets.
 11. A method ofsealing a chamber having a plurality of walls, the chamber being adaptedto accommodate a body part, the method comprising: providing a sealcomprising an opening on a wall of the chamber, wherein the sealcomprises a pair of juxtaposed flexible sheets each having an aperture,the apertures being aligned to form the opening, the sheets being joinedtogether to define a closed space between the sheets; entering a bodypart into the chamber through the opening of the flexible sheets whilethe other body part remaining outside the chamber; and evacuating gasout of the closed space of the seal, wherein upon evacuation of gas fromthe closed space, the seal adopts a substantially rigid form.
 12. Themethod of claim 11, wherein the sheets are joined together by sealingaround the opening and along a portion around the periphery of thesheets.
 13. The method of claim 11, wherein the sheets around theopening is sufficiently flexible to permit expansion of the opening toaccommodate the body part in the chamber in a substantially airtightfit.
 14. The method of claim 13, wherein the flexible sheets are made ofan elastomeric material.
 15. The method of claim 14, wherein the elasticmaterial comprises latex rubber.
 16. The method of claim 11, wherein theseal further comprises a particulate material in the closed space. 17.The method of claim 16, wherein the particulate material comprises beadsof expanded polystyrene.
 18. The method of claim 11, wherein the sealfurther comprises a port in at least one of the sheets to permit gas tocome into or out of the closed space.
 19. The method of claim 18,wherein the port is in the form of a spigot.
 20. The method of claim 11,wherein, the chamber comprises a plethsmograph.